The Composition And Function Of The Bacterial Cell Membrane: Diffusion And Osmosis

Introduction

]Each cell, either prokaryotic or eukaryotic, is characterized by some general features like having a cytoplasm, DNA material and a cell membrane. As it was explained by Bhatia and Lal Ichhpujan (2008), the cell membrane, or the plasma membrane, of the bacterial cell is a fragile structure that separates the cell content from the cell wall. It was also stated that the plasma membrane has a dynamic entity, constantly changing according to the needs of the cell. The cell membrane is an organelle found in all bacterial cells, composed of different elements, carrying out different functions in the cell. [1]: General structure of bacterial cell Collected from Essentials of medical microbiology (2008)

The general structure of the plasma membrane

As demonstrated by Pathak, Tikkoo and Goyal (2016), the cytoplasmic membrane is a selective barrier with the thickness of approximately 8 nm. This highly selective barrier allows the bacterial cell to collect nutrients and excrete waste products. The cell membrane is a phospholipid bilayer structure, where phospholipids molecules are arranged together, setting up most of the membrane. A phospholipid molecule consists of a hydrophilic head(glycerol), which points outwards in the membrane, and hydrophobic tails (fatty acids), pointing inwards in the membrane. Inside the bilayer, they are two types of proteins embedded in between the phospholipid molecules, either being extrinsic proteins or intrinsic proteins. In addition to that, Srivastava family (2003) explained that extrinsic proteins are proteins found on the cell membrane surface, while intrinsic proteins are buried in part or in full within the membrane matrix. Furthermore, they also illustrated that the difference between the eukaryotic cell membrane and bacterial cell membrane, is the lack of sterols. As stated earlier, the cell membrane has a dynamic entity, as both the lipids and the proteins have the ability to move within the matrix. As it was clarified by Pathak, Tikkoo and Goyal (2016) that hydrogen bonds and hydrophobic interactions regulate the overall structure. Cations maintain the structure by forming ionic bonds with negative charges found in the phospholipid bilayer.

Functions of the cell membrane

The plasma membrane in bacterial cells carry out numerous functions that aids the cell to survive . But one of the most famous functions of the cytoplasmic membrane is being a selective membrane that controls what enters and leaves the cell in order to retain all the substances required for metabolism. As it was stated by Kaiser (2019), this mechanism happens through various methods that depends on a number of factors. Furthermore, it was explained that water molecules, dissolved gases like CO2 and O2 and lipid soluble compounds pass across the phospholipid bilayer by diffusion through fine pores that are 0.8nm in diameter. On the contrary, other molecules require carrier molecules in order to cross the membrane. This process occurs in a number of ways including passive diffusion, facilitated diffusion, osmosis and active transport.

Passive Diffusion

As Kaiser (2019) illustrated in his book, passive diffusion is the net movement of uncharged polar compounds and gases across the membrane. This process is passive and doesn’t require any energy to take place. Gas molecules and uncharged polar molecules contains kinetic energy. The unbalanced distribution of the molecules creates a concentration gradient which represents potential energy. Consequently, the molecules move from high concentration to low concentration across the membrane. Therefore, passive diffusion is derived from the potential energy of the concentration gradient.[3]: passive diffusion mechanism. Collected from Microbiology: (Kaiser) (2019)

Facilitated Diffusion

As it was quoted by Kaiser (2019) in his book, facilitated diffusion is the transport of molecules across the plasma membrane with the aid of transport proteins. Transport proteins include uniporters and channel proteins. The molecules move from high concentration to low concentration down the concentration gradient, using the potential energy possessed by the molecules. To begin with, it was illustrated that uniporters are transport proteins that conveys molecules from one side of the membrane to the other. For example, potassium ions K+ enter the bacterial cell using uniporters. In addition to that, channel proteins are classified as the other type of transport proteins that moves water molecules, down the concentration gradient, or certain ions, down an electric potential gradient, from high to low concentration. Usually water molecules move by a process known as osmosis, but aquaporins, channel protein, facilitate its movement across the membrane.

Osmosis

As it was mentioned by Kaiser (2019), osmosis is the movement of water molecules from high water potential to low water potential. As stated before, channel proteins, aquaporins, facilitate the movement of water molecules across the plasma membrane. The aquaporins form channels extending the cytoplasmic membrane as well as transporting water to and from the cytoplasm. However, in order to understand osmosis, the term solution must be understood. A solution is a solute dissolved in a solvent, forming hydrogen bonds with the water molecules. When water is unbound, the molecule is small enough to pass through the membrane while the bounded water molecules are unable to pass. Therefore, the process doesn’t require any metabolic energy [5]: osmosis demonstration

Active Transport

Last but not least, Kaiser (2019) explained that active transport is the mechanism where the cell utilizes both metabolic energy and protein transport to convey molecules through the molecules. It was asserted that active transport is essential to the cell as it enables the bacterial cell to accumulate nutrients, therefore the molecules move from low concentration to high concentration using ATP produced by the cell.

Other functions of the cell membrane

Kaiser (2019) clarified that the bacterial cell membrane carries a lot of functions in order to overcome the shortage of organelles. To begin with, it was found that some plasma membranes can produce energy using either aerobic or anaerobic respiration as well as carry photosynthesis by converting light energy to chemical energy. Moreover, the cell membrane is bounded to the flagella as the motor is located in the cytoplasmic membrane, therefore it is also responsible for motility. In addition to that, the plasma membrane can also be involved with waste removal and endospore formation.

Conclusion

The bacteria plasma membrane is considered one of the main organelles in the bacterial as it carries numerous functions in to order to help the cell survive. It is composed of different components, each with a function.

Reference

1. Essentials of Medical Microbiology: R. Bhatia and R. Lal Ichhpujan (2008) Retrieved from: https://www.pdfdrive.com/essentials-of-medical-microbiology-e33538815.html
2. Introductory Microbiology: D. V Pathak, S. Goyal and A. Tikkoo (2016) Retrieved from : http://ezproxy.bue.edu.eg:2079/eds/detail/detail?vid=2&sid=4f5ea4c5-2908-4007-82c5-f13dde7c2859%40sdc-v-sessmgr03&bdata=JnNpdGU9ZWRzLWxpdmU%3d#AN=edp7854310&db=edspub
3. [2]: photo of fluid mosaic model of the cell membrane is collected from: https://www.biologydiscussion.com/bacteria/bacterial-cells/structure-of-plasma-membrane-with-diagram-bacterial-cell/54737
4. Understanding bacteria: S. Srivastava and P.S. Srivastava (2003) Retrieved from: http://ezproxy.bue.edu.eg:2079/eds/detail/detail?vid=2&sid=f464aaea-5b6f-4935-a2b5-b9cc5808e372%40sessionmgr4008&bdata=JnNpdGU9ZWRzLWxpdmU%3d#AN=edp1889285&db=edspub
5. Microbiology: (Kaiser) (2019) Retrieved from: https://bio.libretexts.org/Bookshelves/Microbiology/Book%3A_Microbiology_(Kaiser)/Unit_1%3A_Introduction_to_Microbiology_and_Prokaryotic_Cell_Anatomy/2%3A_The_Prokaryotic_Cell_-_Bacteria/2.2%3A_The_Cytoplasmic_Membrane
6. Cambridge International AS and A Level Biology Coursework: M. Jones, R. Fosbery, J. Gregory and D. Taylor (2014) Retrieved from: http://aliensservices.com/A%20Level%20Ebooks/Cambridge%20International%20AS%20and%20A%20Level%20Biology%20by%20Mary%20Jones%204th%20Ed.pdf